In the field of hydrocarbon exploration, measurements are often made on reservoir fluids in the flowline of a fluid sampling tool. Such measurements are typically made to obtain information on different reservoir fluid properties, e.g., resistivity, nuclear magnetic resonance (NMR), optical absorption and scattering, dielectric constant, etc. Borehole fluid sampling tools have one or more probes that are pressed against the borehole wall so that reservoir fluids can be pumped out of the earth formations into a flowline situated in the fluid sampling tool. The same tools may also be used to measure reservoir pressures. Fluid sampling tools are widely used in the well-logging industry. Fluid sampling borehole tools are offered by all of the major oilfield service companies. For example, Schlumberger Technology Corporation offers fluid sampling and pressure measurements using a sampling tool called the Modular Dynamics Tester (MDT). Other companies offer fluid sampling tools such as the Reservoir Description Tool (RDT) and the Reservoir Characterization Instrument (RCI). Those tools operate in high pressure and high temperature reservoirs. The pressure of the reservoir fluids in the flowline can exceed 25,000 pounds per square inch, and temperatures can approach or even exceed 200° C. Because of the high temperatures and pressures, the flowlines used in commercial fluid sampling tools are typically made of steel.
However, a metal flowline attenuates electromagnetic (EM) radiation from antennas or other transmitters situated outside of the flowline. The attenuation caused by metallic or highly conductive steel flowlines causes severe signal-to-noise ratio problems for measurements made by EM sensors situated outside of the flowline.
One solution to the attenuation problem caused by steel flowlines is to use sapphire windows embedded on opposite sides of the steel flowline. This is the solution used in the Schlumberger Optical Fluid Analyzer, which is a module within the MDT tool. The windows allow transmission of electromagnetic radiation in the near infrared frequency band through the fluid to measure the optical density of the fluid. However, the steel flowline with embedded sapphire windows is generally not suitable for use when placing an antenna or electromagnetic coil outside of the flowline to make measurements on the fluids because the effects of the steel flowline typically adversely affect the electromagnetic measurements. It has been proposed to place one or more electromagnetic coils inside of a steel flowline; however, the abrasive and corrosive nature of the fluid may damage the coils. Such placement, however, may be useful if the fluid velocity in the flowline is low or temporarily stopped and the fluid is not too corrosive.
Another prior art solution includes a sensor for measuring the resistivity of the fluids in the flowline. The sensor apparatus includes electrodes that are inserted into the fluid through a thick polyetheretherketone (“Peek”) body. This technique has at least two potential drawbacks. First, the electrodes are in the flowline in contact with potentially abrasive and corrosive fluids. Again, this may not be much of a problem if the fluid velocity within the flowline is sufficiently slow and the nature of the fluid is not too corrosive. Second, it is difficult to reliably seal the interfaces between the electrodes and the Peek body. Solving that problem may lead to extra cost or design complexity, but again is not insurmountable. Thus, electrodes and coils disposed in the interior of a steel flowline may be a viable measurement method under the right circumstances.